The atrial natriuretic peptide (ANP) receptor and the homologous c-type natriuretic peptide receptor are homologous yet initiate distinct physiological processes. The ANP receptor is involved in renal and cardiac functions; its activation leads to an increase in natriuresis and diuresis in the kidney and decreased blood pressure. The ANP receptor activating peptide BNP has clinical benefits for congestive heart failure patients. The C-type natriuretic peptide receptor has recently been identified by us as the target for human mutations that cause impaired skeletal growth. Both receptors belong to the family of membrane bound guanylyl cyclases and contain an extracellular ligand binding domain, a transmembrane helix, a kinase-homology domain, a coiled-coil, and guanylyl cyclase domain. Extracellular hormone binding to these receptors leads to the intracellular production of the second messenger cGMP. We have previously determined the 2.0 Angstrom resolution crystal structure of the extracellular ligand binding domain of the ANP receptor. The proposed studies are a continuation of our multi-disciplinary efforts to investigate the mechanism of activation of these receptors. Our first Specific Aim is to investigate whether 1 of the roles of the kinase homology domain is to keep the extracellular domains apart as part of the activation mechanism. This will be tested using structural biology methods with both the intracellular domains as well as with the full length ANP receptor. The second Aim is to test the hypothesis that the domains of the ANP receptor are correctly positioned, and optimized for length. This will be probed by insertional and deletion mutagenesis studies targeting the inter-domain regions. The third Specific Aim is to determine whether the transmembrane helices are involved in lateral movements within the membrane during the activation mechanism. The fourth Aim is to test the hypothesis that a number of the genetic defects in the c-type natriuretic peptide receptor involve mutations that affect the signal transduction mechanism of these receptors. These proposed studies on natriuretic peptide receptors will increase our understanding of mechanisms of activation and function, and could enhance the development of new renal, cardiovascular, or skeletal growth stimulatory drugs.